The present invention is directed to forming a pattern on a photosensitive substrate, such as used in the integrated circuit manufacturing industry, and to the apparatus for producing the pattern. More particularly, the present invention is directed to forming a pattern on a photosensitive coating in a step-and-scan lithography system, and includes a process and apparatus to adjust the exposure dose in a localized area by use of a segmented slit.
Modern silicon technology uses photolithography techniques to manufacture integrated circuits (IC""s) via photomasks. The photomasks are typically formed of an opaque pattern in a material such as chrome which is protected, by pellicles, from scratching and contamination. A pellicle is a thin sheet of clear material that is attached to the patterned material of the photomask so as to prevent foreign matter from contaminating the mask surface itself and thereby distorting the mask pattern. The pellicle precludes contaminating debris from the focal plane of the photomask pattern.
The substrate on which an integrated circuit is formed is called a wafer. Wafers are divided into individual integrated circuit sites called die. The critical dimensions of features of integrated circuits produced in the modern integrated circuit manufacturing industry are very small and continue to shrink. As these critical dimensions shrink, the degree of device complexity can be increased for a given die size.
The popular approach in photolithography is to expose each integrated circuit site (die) with a single mask pattern, then move to the next site, expose that site, and then move on repeating this process until each site has been exposed on the wafer. This is called xe2x80x9cstep and repeat.xe2x80x9d When a mask with a single pattern in a transfer region is used in a step-and-repeat process, the mask or photomask is referred to as a xe2x80x9creticle.xe2x80x9d Such use is favored because a one-to-one size ratio between the mask pattern and the pattern formed on the wafer is not favored, and modern photolithography equipment permits a mask image to be reliably, accurately, and repeatably reduced as it is formed on a substrate with a photosensitive coating. This procedure allows the mask pattern to be larger than the pattern actually produced on the integrated circuit. The mask image may be reduced by a factor which is commonly four or five times. For example, the image may be xe2x80x9cstepped-down 4xc3x97xe2x80x9d by a lens as the wafer site is exposed. As a result, the defects due to particle contamination, scratches, and other sources of defects on the mask surface are reduced, which correspondingly diminishes their deleterious effects.
In actual practice, control of the light exposure dose is important. A commonly implemented approach involves placing an opaque or light-interrupting barrier, having an aperture, between the light source and the mask or reticle. The aperture may be in the form of a slit. The slit is moved across the mask, thereby scanning the mask or reticle image from the transfer region and onto a site on the wafer, and exposing the photosensitive material coated on the wafer surface. After this scan process is completed at one site, the system is stepped over to another site on the wafer or substrate and the scan process is repeated. This is called the xe2x80x9cstep-and-scanxe2x80x9d procedure.
When the exposure area or aperture formed within a light-interrupting member is a slit, the orientation of the slit is typically perpendicular to the scanning direction. For a given location along the length of the slit, the exposure dose projected onto the substrate being exposed depends upon the intensity of the light and the time the light source is projected through the slit and onto the substrate. For a given slit width, the exposure dose achieved on the substrate surface is increased as the scan speed is decreased. Conversely, for a given scan speed, the exposure dose achieved on the substrate surface is increased as the width of the slit is increased because the illumination impinges upon the substrate for a longer period of time.
In a step-and-scan system as described above, a number of problems exist. One relatively minor problem involves maintaining the intensity of the light dosage uniformly on the photosensitive substrate (wafer) throughout the exposure region. Unfortunately, the projection optics or illumination source may have non-uniformities which cause variation in the light dosage at the wafer surface. Although this particular variance due to the non-uniformity may be small, the critical nature of the application makes any non-uniformity significant and correction highly desirable.
Second, there may be non-uniformities in the processing used to produce the masks. Reactive ion etching (RIE) effects at the peripheries of individual die are such an example. These non-uniformities are static to the individual mask, but vary from mask-to-mask. The problem associated with this non-uniformity is that, for a given mask level, the exposure dosage would have to be adjusted throughout the scan based upon the specific mask used, which is undesirable. Suitable control of the exposure dose would allow compensation for these repeatable mask-to-mask non-uniformities.
A third problem is also related to the nature of mask manufacturing: some pattern defects may be present in the mask. Many of these pattern defects are repaired before the mask is put into use. There are defects, however, which as a practical matter cannot be repaired. This is generally due to the small size of the defects and also to proximity effects with respect to the other mask features. Appropriate control of the exposure field can partially correct for some of the pattern defects which fall into the category of uncorrectable mask defects.
Lastly, additional defects are introduced onto the masks, because the masks themselves are prone to problems associated with handling, including contamination and scratches. As above, appropriate control of the exposure field could also correct for many of the defects associated with mask handling. Furthermore, the elimination of masks by using a mask-less, direct-write optical photolithography system would circumvent all of the above problems associated with the photomasks or reticles.
To correct for the problems inherent in conventional systems and procedures, the present invention provides a process to dynamically adjust the exposure dose which reaches the photosensitive substrate at one position in the exposure field relative to other positions. The adjustment is made in response to system data obtained regarding mask defects and other system non-uniformities. The present invention addresses the shortcomings of the prior art by providing an adjusted exposure dose within the exposure field for the purpose of correction. The present invention provides an exposure apparatus having an exposure field with individually controllable features which provide for an adjusted light dosage within the exposure field.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.